Grav can not find vendor.php, will not load

I have a local version of my Grav site for testing on my Windows 10 laptop. It was working fine for a while. However, in the last few weeks, whenever I try to view it in a browser, I get the following errors:

**Warning** : require(C:\Users\xxxxxx\Dropbox\www\xxxxxxxxxxxx\site/vendor/autoload.php): failed to open stream: No such file or directory in **C:\Users\xxxxxx\Dropbox\www\xxxxxxxxxx\site\index.php** on line **34**

**Fatal error** : require(): Failed opening required 'C:\Users\xxxxxx\Dropbox\www\xxxxxxxxxx\site/vendor/autoload.php' (include_path='.;C:\php\pear') in **C:\Users\xxxxxx\Dropbox\www\xxxxxxxxxxxx\site\index.php** on line **34**

I have checked, and there the file vendor/autoload.php is there. I can also run infophp() and see results, so PHP is running. My other non-Grav sites work, so I know this is a Grav issue of some kind.

Why is Grav not loading?